Northeastern Syria, where the Turkish aggression took place, has been controlled by the Syrian Democratic Forces led by the Kurdish People's Protection Units.
They are the key ally of the U.S. in the battle against ISIS, and the U.S. had been stationing its troops in the area to maintain a "no-fly zone" protecting it from external attacks.
The major threat was from Turkey.
As the area is on the border between Turkey and Syria, Turkey had been aiming to remove the Kurdish forces from the area and create a "safe zone" to transfer millions of Syrian refugees.
It had been negotiating with the U.S. to accomplish the goal, but the two sides failed to reach a deal.
Against this backdrop, the U.S. had been keeping its forces in the area to deter Turkish attacks against a key ally in the fight against ISIS.
However, that policy changed a couple of days of ago when President Trump announcing the U.S. was withdrawing its troops from the area.
Just four days after the withdrawal, Turkey took military action, largely driven by its own national interest.
Such a move was not unexpected following the removal of U.S. protection for the Kurdish.
Turkey, meanwhile, insists its aggression does not violate the international law, and claims the Kurdish are a "terrorist" group.
Lee Kyung-eun, Arirang News.